Onchocerciasis

Essentials of Diagnosis

• Conjunctivitis progressing to blindness

• Severe pruritus; skin excoriations, thickening, and depigmentation; and subcutaneous nodules

• Microfilariae in skin snips and on slit-lamp examination; adult worms in subcutaneous nodules

General Considerations

• Caused by Onchocerca volvulus

• Infection transmitted by simulium flies (blackflies), which breed in fast-flowing streams and bite during the day

• After the bite of an infected blackfly, larvae are deposited in skin, where adult worms develop over 6–12 months

• Adult worms live in subcutaneous connective tissue or muscle nodules for a decade or more

• Microfilariae are released from nodules and migrate through subcutaneous and ocular tissues

• Disease is due to responses to worms and to intracellular Wolbachia bacteria

Demographics

• An estimated 37 million persons are infected, of whom

  • 3–4 million have skin disease

  • 300,000 are blinded

  • 500,000 are severely visually impaired

• Over 99% of infections are in subsaharan Africa, especially the West African savanna

• Half of the cases are in Nigeria and Congo

• In hyperendemic African villages,

  • Close to 100% of persons are infected

  • 10% or more of the population is blind

• Disease also prevalent in

  • Southwestern Arabian peninsula

  • Latin America (eg, Mexico, Guatemala, Venezuela, Colombia, Ecuador, Northwestern Brazil)

Symptoms and Signs

• Incubation period of up to 1–3 years

• Inguinal and femoral lymphadenopathy

• Systemic symptoms include weight loss and musculoskeletal pain

• Skin

  • Erythematous, papular, pruritic rash, which may progress to chronic skin thickening and depigmentation

  • Itching may be severe and unresponsive to medications

  • Numerous firm, nontender, movable subcutaneous nodules of about 0.5–3 cm contain adult worms

• Eye

  • Microfilariae migrating through the eyes elicit host responses that lead to pathology

  • Findings include punctate keratitis and corneal opacities

Differential Diagnosis

• Glaucoma

• Loiasis (Loa loa infection)

• Gnathostomiasis

• Cysticercosis (with ophthalmic involvement)

• Sporotrichosis

• Coccidioidomycosis

Laboratory Tests

• Complete blood count; eosinophilia is a common but inconsistent finding

• Serologic tests

  • Poorly standardized

  • Nonspecific

  • Do not distinguish current from past infection

Imaging Studies

• Ultrasound may identify characteristic findings suggestive of adult worms in skin nodules

Diagnostic Procedures

• Diagnosis is made by identifying microfilariae in

  • Skin snips from the iliac crest in persons from Africa or from the scapula in persons from the Americas

  • Cornea or anterior chamber by slit-lamp examination

  • Urine (rare)

• Skin snips stand in saline for 2–4 h or longer before examination

• Identifying adult worms in biopsy or aspirate of a nodule also confirms diagnosis

• Deep punch biopsies not needed

• The Mazzotti test uses 50 mg of diethylcarbamazine

  • Should be used only after other tests are negative

  • Can elicit severe skin and eye reactions in persons with heavy infection

  • Positive results (exacerbation of skin rash and pruritus) usually occur within 3 h

• A related and safer test using topical diethylcarbamazine available in some areas

• Antigen and antibody detection tests are under study

Medications

• Ivermectin

  • Drug of choice

  • Kills microfilariae but not adult worms, so repeat administrations are required

  • Initial dose: 150 mcg/mL orally

  • Re-treatment schedules

    • Not standardized

    • One regimen is to treat every 3 months for 1 year, followed by every 6–12 months for suspected lifespan of adult worms (about 15 years)

  • Side effects include

    • Fever

    • Pruritus

    • Urticaria

    • Myalgias

    • Edema

    • Hypotension

    • Tender lymphadenopathy

  • Use with caution in patients at risk for loiasis because can elicit severe reactions (eg, encephalopathy)

• Moxidectin

  • Is FDA-approved for the treatment of onchocerciasis

  • Well-tolerated

  • Superior to ivermectin in suppressing skin microfilariae

• Doxycycline

  • 100 mg/day for 6 weeks kills Wolbachia bacteria and prevents parasite embryogenesis for at least 18 months

  • May become first-line agent due to improved activity against adult worms compared to other agents and limited toxicity

Complications

• Vision loss and blindness from

  • Sclerosing keratitis

  • Iridocyclitis

  • Glaucoma

  • Choroiditis

  • Optic atrophy

Prevention

• Avoidance of biting flies

• Major efforts underway to control insect vectors in Africa

• Mass distribution of ivermectin for intermittent administration at community level is ongoing; prevalence of severe skin and eye disease is decreasing

When to Refer

• All patients should be referred to a clinician who has expertise in the diagnosis and treatment of this disease

• All patients with ocular involvement should be referred to an ophthalmologist